Several Doppler phantoms have been described in the prior art. These fall into two main categories.
In the first group are phantoms which include a moving target such as a ball bearing or suspended string. The moving target is used to map the location and size of the sensitive Doppler sample volume of the ultrasound device. In this category, for example, Walker et al, "Evaluating Doppler Devices Using a String Target," Journal of Clinical Ultrasound, 10:25, 1982, describe a method in which a pair of moving strings at two different depths is used in a test target to measure Doppler sample volume size, sample location, and amplitude sensitivity to Doppler shift. An alternative method, described by Hoeks et al, "Methods to Evaluate the Sample Volume of Pulsed Doppler Systems", Ultrasound in Medicine and Biology, 10:427, 1984, is to couple a small target such as a sphere to a vibrating loudspeaker and then map the Doppler sample volume using this moving target.
In the second category of Doppler phantoms, the objective is to simulate blood flow, using a blood-mimicking liquid, through a simulated blood vessel in a tissue-mimicking material. Phantoms of this kind typically consist more or less of a solid block of tissue-mimicking material, such as gelatin or polymer, which contain tubes of varying diameters and bifurcations. Such phantoms seek to simulate blood flow through abdominal and peripheral vessels. Examples of such devices, of varying degrees of sophistication, are taught by Newhouse et al, "A Proposed Standard Target for Ultrasound Doppler Gain Calibration", Ultrasound in Medicine and Biology, 8, 313-316, 1982; McDicken, "A Versatile Test-Object for the Calibration of Ultrasonic Doppler Flow Instruments", Ultrasound in Medicine and Biology, 12:245, 1986; and Boote et al, "Performance Tests of Doppler Ultrasound Equipment with a Tissue and Blood Mimicking Phantom", Journal of Ultrasound Medicine, 7, 137-147, 1988. These phantoms allow independent measurement of "blood" flow and enable calibration of the fluid velocity estimations of Doppler ultrasound. Commercial versions of these test objects, "Tissue Mimicking Ultrasound Phantom" Model 409, Radiation Measurements, Inc., Middleton, Wis., 53362 (U.S. Pat. No. 4,277,367); "Ultrasound Doppler Phantom", ATS Laboratories, Inc., Bridgeport, Conn., 06608; and "Ultrasound Doppler Phantom", Interspec, Inc., Lewiston, Pa., 17044, are also available.
The most extensive application of medical Doppler ultrasound, however, is in cardiac diagnosis. Doppler ultrasound is used to measure blood flow through cardiac valves and thus to estimate pressure drops across individual heart valves for detection of valve dysfunctions. In addition, cardiac Doppler ultrasound is used to detect anatomical anomalies such as ventricular septal defects. The field of cardiac ultrasound imaging and Doppler ultrasound has a known and long-standing need for an anthropomorphic cardiac Doppler phantom to assess the performance of Doppler ultrasound under more realistic clinical conditions. Limited attempts to develop a Doppler phantom for cardiac applications include a modified cardiac pulse duplicator, "Cardiac Pulse Duplicator", Model MP1, Dynatek Laboratories, Annandale, N.J., 08801 as described by Cary et al in a private communication. However, this device is fabricated from rigid plastic, which generates strong acoustic reverberations unsuitable for diagnostic ultrasound examinations. In addition, such a modified pulse duplicator shows no similarity to the contracting muscular heart of human anatomy. Other examples include a modified cardiac pulse duplicator, "Valve Visualization Pulse Duplicator", Model MV/T1 Dynatek Laboratories, Annandale, N.J., 08801, as described by Gels et al, "In Vitro Ultrasound Flow Imaging Through Prosthetic Heart Valves", Medical Instrumentation, 21(2), 66-74 , 1987, that includes a soft rubber vessel to simulate the aorta downstream of the aortic valve. This system, using microbubbles in tap water as an ultrasound contrast agent, enables flow visualization with diagnostic ultrasound imaging equipment to evaluate, qualitatively, the performance of prosthetic heart valves in vitro but does not permit simulation of the contracting heart of human anatomy.
In addition, Reul, "Cardiovascular Simulation Models", Life Support Systems, 2, 77-98, 1984, has developed hydraulically driven cardiovascular simulation models for the evaluation of prosthetic heart valves, using pressure transducers, flow transducers, and optical video filming of suspended particles in an aqueousglycerol solution, but the cardiac models themselves are housed in rigid polymethylmethacrylate boxes. There are no viewing ports in the containers for ultrasound imaging or ultrasound Doppler studies. The polymethylmethacrylate containers produce high attenuation and reverberations that are unsuitable for ultrasound imaging or ultrasound Doppler studies. The hydraulic drive unit is mounted below the left ventricle in this device and precludes an apical ultrasound view, one of the most common views in clinical cardiac diagnosis. The hydraulic medium surrounding the left ventricle also is not specified. However, the attenuation scattering and velocity values of this medium are critical to the development of an ultrasound phantom as will be described below. Ordinary or distilled water, for example, is not suitable.
The field of cardiac assist devices, furthermore, includes left ventricular assist devices (LVAD) that typically have pneumatic pumps composed of a rigid plastic shell surrounding the flexible polymer left ventricle, Van Citters et al, "Artificial Heart and Assist Devices: Directions, Needs, Costs, Societal and Ethical Issues", Artificial Organs, 9(4), 375-415, 1985. The presence of air around the blood sac and the hard plastic shell make this technology unsuitable for a cardiac ultrasound phantom. One hydraulically driven LVAD is known which also consists of a rigid case surrounding the left ventricle, Altieri et al, "Implantable Ventricular Assist Systems", Artificial Organs, 11(3), 237-246, 1987. Again, however, this shell precludes use of the LVAD as an ultrasound phantom.
A need clearly exists in the art of Doppler and ultrasound imaging and the like for medical purposes for an anthropomorphic ultrasound phantom that realistically enables ultrasound analyses of simulated defects in human hearts and provides reverberation free outputs suitable for comparison of different devices and simulation studies.